EXPERIMENTAL DEPOSITION OF ARAGONITE MUD FROM MOVING SUSPENSIONS

Flume experiments on clay deposition have established that clays can be deposited from swift moving suspensions. At flow velocities that transport and deposit sand (20-35 cm/s, 5 cm flow depth) clay suspensions produce deposition-prone floccules that form migrating floccule ripples. Clay beds that form when these ripples accrete have a finely laminated appearance once compacted.

We used a racetrack flume with paddle-belt drive (to avoid destruction of floccules) to repeat these flume experiments with synthetic and natural aragonite mud (bleached to remove organic coatings) in water of 35 ‰ salinity. We observed the same pattern of flocculation, ripple formation, and bed accretion as seen previously in our experiments with clays. Aragonite mud forms somewhat larger floccules than kaolinite and smectite suspensions under otherwise comparable conditions, and the critical velocity of sedimentation for aragonite mud is approximately 5 cm/s above that observed for clays. The size of aragonite bedload floccules increases systematically as flow velocity and shear stress is reduced.

The aragonite mud beds that form as a consequence of floccule ripple accretion appear parallel laminated at first glance, but reveal internal low angle cross-lamination on closer inspection. In plan view, the observed pattern of ripple foresets is identical to rib and furrow structure in sandstones.

Just as previously assumed for terrigenous muds, there has been a long-standing notion that abundant carbonate mud accumulation reflects quiescent conditions of offshore and deeper water environments. Our experiments clearly demonstrate that carbonate muds can also accumulate in more energetic settings. In the sedimentary record of carbonate rocks, interbedded grainstones and micrites may thus not necessarily reflect significant shifts in depositional energy (or water depth), but alternatively may imply a shift in supplied sediment type. Thus, our results suggest that published interpretations of ancient micrites and derived paleoceanographic conditions may need to be reevaluated.